<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin:0;
            padding:0;
        }
        .container,.left,.main {
            height: 100vh;
        }
        .container {
            background-color: red;
            display: flex;
        }
        .left {
            background-color: green;
            width: 300px;
        }
        .main {
            background-color: yellow;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="left"></div>
        <div class="main"></div>
    </div>
</body>
</html>